CDMA systems and protocols are specified in Interim-Standard ninety-five (IS-95) by the US Telecommunications Industry Association and CDMA2000 RC1.
The link between a mobile station and a base station is made up of two channels, the forward channel (from the base station to the mobile station) and the reverse channel (from the mobile station to the base station).
The forward channel is composed of four different types of code channels: a pilot channel, sync channel, paging channels and forward traffic channels.
A typical forward CDMA channel consists of 64 code channels. The 64 code channels consist of a pilot channel, one sync channel, seven paging channels and 55 forward traffic channels.
The pilot channel is an unmodulated, direct-sequence spread spectrum signal that is transmitted at all times by the base station on every active forward channel. The mobile station monitors the pilot channel to acquire the timing of the forward CDMA channel and to obtain a phase reference for coherent demodulation.
The sync channel is used to transport synchronisation messages to mobile stations within a CDMA cell. It is used by the mobile station to acquire initial time synchronisation.
The paging channel is used to transmit control information and pages to mobile stations residing in the CDMA cell.
The forward traffic channel is used for transmission of user and signalling traffic from the base station to a specific mobile station during a phone call.
All of the code channels in the forward CDMA channel are orthogonally spread by an appropriate Walsh function and then undergo quadrature spreading (the sync channel, paging channel and forward traffic channel also undergo several other processes before being orthogonally spread).
In particular, long codes which are pseudo-noise PN sequences, are used to scramble the paging and traffic channels. Each channel is uniquely assigned a long PN code which has a period of 242−1 chips. The long code is specified by the characteristic polynomial p(x)=x42+x35+x33+x31+x27+x26+x25+x22+x21+x+19+x18+x17+x16+x10+x7+x6+x5+x3+x2+x1+1. Each PN chip of the long code is usually generated by inner product of a 42 bit mask (a code exclusive to the channel) and the 42 bit state vector of a linear sequence generator.
In the prior art, a PN sequence is usually generated by a linear PN s quence gen rator which consists of a 42 stage linear feedback shift r gister where the feedback logic is exclusive-OR (XOR) gates. Binary sequences are shifted through the shift registers in response to clock pulses, and the output of the various stages are logically combined and fed back as the input to the first stage. A 42-stage linear shift register generates a maximal length PN sequence of 242−1 symbols.
Further details of PN sequence generation can be found by reference to IS-95 or by referring to text books such as Wireless Communications Principles & Practice by Theodore S. Rappaport (ISBN 0-13-375536-3).
It will be appreciated that in order to achieve the IS-95 PN sequence chip rate of 1.2288 million chips per second, the shift register must be updated approximately every 19.5 microseconds. A single chip of the PN sequence must be produced approximately every 52 microseconds.
The forward CDMA channels are all orthogonally spread using Walsh Functions at a fixed chip rate of 1.2288 Mcps. The purpose of orthogonal spreading is to provide orthogonal channelization among all code channels. The pilot channel is always spread with Walsh code 0, the SYNC channel is always spread with Walsh code 32 and the paging channels are assigned Walsh codes 1 through 7.
Following Walsh spreading, all code channels undergo Quadrature Spreading, which involves performing the modulo-2 addition of the orthogonally spread data with the short code pseudo noise (PN) sequence generated by the short code generator. The tap polynomials for the Short Code shift registers are:PI(x)=x15+x13+x9+x8+x7+x5+1PQ(x)=x15+x12+x11+x10+x6+x5+x4+x3+1
The output of the quadrature spreader is in the form of In-Phase (I) and Quadrature (Q) channels. The I and Q data streams (channels) are then each passed through a Baseband Filter which shapes the waveform to meet required bandwidth constraints as well as minimize Inter Symbol Interference (ISI).
The I and Q channels are then modulated using Quadrature Phase Shift keying (QPSK). This is done in the I channel by amplitude modulating the cosine function with an amplitude of binary 0's and 1's to produce a BPSK (Binary Phase Shift Keying) waveform. In the Q channel, the sine function is modulated producing an orthogonal BPSK waveform. The summation of the BPSK waveforms then yields the QPSK waveform.
To the applicant's knowledge all existing CDMA systems are implemented as hardware. Recently, it has been proposed that existing cellular telecommunications hardware could be replaced by a software implemented radio telecommunication system. It will be appreciated that software-implementations will benefit from an efficient technique for updating the registers holding the long and short codes.